The present invention relates to an apparatus and a method for measuring a flatness of a thin plate, which measures a flatness of surface or a thickness variation of a thin plate such as a wafer for manufacturing a semiconductor, a magnetic recording disc or the like.
The wafer for manufacturing the semiconductor is constituted by a thin plate such as a silicone or the like. In order to form a semiconductor device or a circuit on a wafer surface, there is executed a working process to which a photo engraving technique, a printing technique, various kinds of micro-fabrication techniques or the like is applied. In the wafer to which such working process is applied, it is important to increase a flatness of a surface. When the flatness of the wafer is deteriorated, a pattern of the device or the circuit is unclearly formed, or a profile of a material to be printed on the wafer surface in a pattern shape becomes indefinite, at a time of photo engraving. In particular, as a densification or a large-size of the semiconductor device or the circuit is promoted, the problem mentioned above becomes remarkable.
In a semiconductor manufacturing step, various kinds of processes are frequently executed in a state in which a whole surface of the wafer is supported in a closely contact manner to a flat supporting surface by a means such as a vacuum adsorption or the like. At this time, in the case that a flatness or a thickness of the wafer has a dispersion, the dispersion of the thickness appears as a dispersion of the flatness of the wafer surface as it is at a time of supporting the wafer to the flat supporting surface in a closely contact manner. Accordingly, it is required that the dispersion is not generated in the flatness or the thickness of the wafer. In order to estimate whether or not the thickness variation of the manufactured wafer is large in the manufacturing step of the wafer or the like, it is necessary to accurately and efficiently measure the flatness and the thickness variation of the wafer.
As a conventional wafer thickness variation measuring apparatus, there is a structure described in Japanese Patent Application Laid-Open No. 2000-28372. In this measuring apparatus, a displacement of the wafer surface is measured by optical probes arranged in side portions of both surfaces of the wafer while rotating a disc-like wafer in a perpendicularly standing state, whereby a magnitude of the thickness variation of the wafer is calculated on the basis of this displacement. By scanning the optical probes in a radial direction of the wafer, it is possible to measure the thickness variation with respect to the whole surface of the wafer.
The optical probe used in the measuring apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-28372 is structured such that a spot diameter of a laser beam focused by the lens is about 0.1 mm, and has an advantage that a spatial resolving capacity for measurement is high. This is an excellent probe which can measured a fine external waviness on the wafer. However, since the spot diameter is small, it is hard to measure a whole surface of the wafer all over. When it is intended to scan the whole surface of the wafer having a diameter 300 mm all over by the spot diameter of 0.1 mm, it is necessary to set a measurement pitch to be equal to or less than 0.1 mm and rotate at 1500 times at the minimum, in view of computation. On the contrary, when the spot diameter is set to 1 mm, it is possible to scan the whole surface of the wafer only at 150 rotations, however, the spatial resolving capacity is reduced.
An object of the present invention is to provide an apparatus and a method for measuring a flatness of a thin plate which can measure a flatness at a high accuracy around a wide range of the thin plate by a comparatively large spot diameter between 0.5 mm and 2 mm.
In order to achieve the object mentioned above, in accordance with the present invention, there is provided an optical probe measuring apparatus, said apparatus having an optical probe comprising:
a polarization beam splitter separating a laser beam emitted from a laser generator and transmitting through an isolator into a measurement light and a reference light, irradiating the measurement light to a measurement surface of a thin plate and irradiating the reference light to a reference surface arranged perpendicular to the measurement surface;
a quarter wavelength plate arranged between the polarization beam splitter and the measurement surface and through which the measurement light transmits;
a quarter wavelength plate arranged between the polarization beam splitter and the reference surface and through which the reference light transmits;
a focusing and reflecting means for focusing and reflecting the measurement light reflecting by the measurement surface and reflecting by the polarization beam splitter, and the reference light reflecting by the reference surface and transmitting through the polarization beam splitter;
a half mirror reflecting the measurement light and the reference light which return from the polarization beam splitter; and
a light receiving portion receiving the measurement light and the reference light which are reflected by the half mirror so as to interfere, converting an interference light intensity change into an electric signal, and counting the electric signal so as to measure a flatness of the measurement surface.
In the optical probe measuring apparatus mentioned above, the laser beam which is not focused by the lens has a spot diameter of about 1 mm, and the laser beam is separated into the measurement light and the reference light by the polarization beam splitter so as to be irradiated to the measurement surface and the reference surface, respectively. The respective reflection lights are again irradiated to the measurement surface and the reference surface, and are reflected. These reflection lights of the measurement light and the reference light return to an emitting light path of the laser beam, and are injected to the light receiving portion. In this case, since the measurement light and the reference light are interfered on the basis of a light path difference between the measurement light and the reference light due to a distortion or an unevenness existing on the measurement surface, the flatness of the measurement surface can be measured. When irradiating the measurement light to the measurement surface, the reflection light reflects in a different direction from that of the incident light path due to an incline change of the measurement surface, however, since this is again irradiated to the measurement surface, the reflection light returns to the same direction as that of the initial incoming light. Accordingly, it is possible to interfere the reflection light with the reference light even when the measurement surface is in some degree inclined.
It is preferable that the focusing and reflecting means is constituted by a focusing lens focusing the measurement light and the reference light and a mirror arranged at the focusing position. Otherwise, the focusing and reflecting means can be constituted as a corner cube prism reflecting the measurement light and the reference light.
It is possible to obtain a spot diameter of 0.5 to 2 mm by arranging a beam expander in the emitting light path of the laser generator.
It is preferable that the optical probe is arranged in both sides of the thin plate in an opposing manner, and a measuring means for adding the measurement data concerning the flatness so as to measure a thickness variation of the thin plate is provided.
In order to achieve the object mentioned above, in accordance with the present invention, there is provided an optical probe measuring method comprising:
separating a laser beam into a measurement light and a reference light by a polarization beam splitter;
irradiating the measurement light to a measurement surface of a thin plate via a quarter wavelength plate;
irradiating the reference light to a reference surface arranged perpendicular to the measurement surface via a quarter wavelength plate;
focusing and reflecting the measurement light reflecting by the measurement surface, transmitting through the quarter wavelength plate and reflecting by the polarization beam splitter, and the reference light reflecting by the reference surface, transmitting through the quarter wavelength plate and transmitting through the polarization beam splitter;
again separating the measurement light and the reference light which focus and reflect, by the polarization beam splitter, and irradiating the measurement light and the reference light respectively on the measurement surface and the reference surface via the quarter wavelength plates;
reflecting the measurement light which again reflects by the measurement surface, transmits through the quarter wavelength plate and transmits through the polarization beam splitter, and the reference light which again reflects by the reference surface, transmits through the quarter wavelength plate and reflects by the polarization beam splitter, by a half mirror; and
receiving the measurement light and the reference light which are reflected by the half mirror so as to interfere, converting an interference light intensity change into an electric signal, and counting the electric signal so as to measure a flatness of the measurement surface.
It is preferable to execute the optical probe measuring method in both sides of the thin plate, and add these measurement data so as to measure the thickness variation of the thin plate.